This invention relates to III-V semiconductor materials, and more particularly relates to processes for producing III-V nitride substrate materials.
As the practical realization of the theoretical limits of well-developed semiconductors such as silicon are approached, interest in wide-bandgap semiconductors, and particularly III-V nitrides, is increasing for a wide range of electrical and optical device applications. The III-V nitrides, e.g., InN, GaN, and AIN, and their solid solutions, are unique among compounds in that they all are characterized by relatively large and direct bandgaps, extreme structural stability, high electron saturation velocities, and high thermal conductivities. These electrical and mechanical qualities of the III-V nitrides have been exploited in the development of, e.g., high-power light emitting diodes (LEDs) in the yellow to ultraviolet spectral range, with blue LEDs now a commercial reality. In addition, III-V nitride injection laser diodes, ultraviolet detectors, and high-power, high-temperature field-effect transistors have been demonstrated and are attracting increasing developmental efforts.
Producibility and reproducibility of the III-V nitride materials used to fabricate these devices are currently the most significant challenges in their development. Perhaps most significantly, bulk substrates of III-V nitride single crystal material are not yet available, and thus quasi-substrate, epitaxial III-V nitride layers must be grown on a foreign growth substrate. But there currently exists no lattice-matched material ideally suitable as a foreign growth substrate. As a result, III-V nitride film growth is typically carried out on a convenient foreign substrate such as sapphire, even though sapphire has a lattice constant significantly different from that of the III-V nitrides.
III-V nitride film growth directly on sapphire or other foreign substrate has been found to result in cracking of the nitride film at film thicknesses above some relatively thin minimum thickness. It is well-recognized, however, that the film quality increases with increased film thickness. In an effort to suppress the cracking condition, it has been suggested to grow the nitride film not on a bare foreign substrate but instead to grow the nitride film on a buffer layer deposited on the substrate. For example, GaN growth processes have been demonstrated in which a GaN layer is grown on a layer of AIN or ZnO which is first deposited on a sapphire growth substrate.
While such growth buffer layers have been shown to somewhat reduce nitride layer cracking such that thicker III-V nitride layers can be grown, they have not been entirely successful, and have not significantly addressed the challenge of further improving the electrical and structural quality of thicker epitaxial layers. One particularly important structural characteristic of the epitaxial layer to be improved is the density of dislocations in the layer; the relatively high dislocation density typical of III-V nitride layers directly impacts the electrical performance of devices fabricated in the layer. For example, dislocations can serve as optical scattering centers in coherent light emitting devices, requiring a higher laser threshold current density. Dislocations also can introduce deep defect energy levels which increase the leakage current of electrical devices fabricated in the layer. Electrical device stability can also be limited at high injection currents due to diffusion of impurities, such as metals, along dislocations.
The degree of surface smoothness of an epitaxial nitride layer is also a critical structural characteristic. Nitride layers having surface morphology such as facets impede the ability to carry out photolithographic processes on the layers for producing devices, and can degrade laser performance by, e.g., broadening the laser emission spectra. Although it is generally r recognized that a higher quality III-V nitride layer can be produced by growing a second or more nitride layers over a first nitride epitaxial layer grown on a foreign growth substrate, it is also well-recognized that the quality of the first nitride epitaxial layer is critical to the quality of any overgrowth nitride layers.
Beyond these considerations for III-V nitride layer quality, the epitaxial growth of such layers on a foreign growth substrate imposes limitations on the operation of devices fabricated in the layers, given that the grown layer remains attached atop the growth substrate. For example, sapphire, the historically most common growth substrate, is electrically insulating, and thus electrical contact to devices fabricated in the nitride layer atop the sapphire substrate can not be made to the backside of the nitride layer through the substrate. More complicated one-sided contact scenarios are thus typically required. In addition, the poor thermal conductivity of sapphire severely limits the ability to adequately cool electrical devices in the upper nitride layer. This poor thermal conductivity is in contrast to the very high thermal conductivity of nitrides such as GaN, and thus impedes the ability to exploit that characteristic of the nitrides.
As advances in III-V nitride-based device applications continue to be made, it is clear that fundamental improvements in the quality and structure of III-V nitride epitaxial layers are necessary to enable the practical realization of the devices and applications.